Ecologie van Fluiters Phylloscopus sibilatrix in Nederlandse bossen

A study of the breeding biology of Wood Warblers in western Drenthe in the northern Netherlands (4500 ha, of which 2900 ha forested) in 2012 was expanded with long-term data on phenology and trends in the same region, and on the Veluwe (7150 ha of woodland) and Planken Wambuis (1277 ha of woodland) in the central Netherlands. Wood Warbler numbers showed wide annual variations, with a factor of 9-14 in two regions in Drenthe (1984-2012) and a factor of 17-41 in two study plots on the Veluwe (1974-2012). Numbers since the mid-1970s can be considered rather stable except for an upsurge of short duration in the early 1990s, the latter recorded in many different regions throughout the country and perhaps the reason why monitoring schemes suggest a decline (having started around the apex in the 1990s). Mean densities ranged between almost nil up to 7.5 pairs/100 of woodland, depending on region and year. Fluctuations in numbers were rather synchronized between regions, especially during peak years like 1990 and 1993. In 2012, 22 out of 34 territories were occupied by a solitary male in Drenthe. Of twelve paired males, two were presumably polyterritorial (not based on colour-ringed individuals, but based on behaviour and singing schedules of males within 250 m of a territory with a breeding female). Spring arrival on the breeding grounds, here exemplified as mean arrival date of the first three males, showed an advancement in both Drenthe (1990-2012) and Veluwe (1968-2012), from early May in the 1970 s to mid/late April in later decades. Egg-laying in Drenthe in 2012 commenced around mid-May. Mean start of laying in 11 pairs was 22 May (SD=4.l2, range 14 May-22 June), and two pairs starting in late June are considered to have been repeat layings. Of 12 clutches, only three resulted in (13) fledglings. One clutch failed during the egg stage, eight during the nestlings stage. Causes of failure were slugs (6x, of which two cases could be unambiguously attributed to Arion ater. invading the nest and covering the chicks in a thick layer of slime), mice (2x) and unknown (lx). Clutch size declined from 6 in mid-May to 5-6 in late May/early June and to 4 in late June (7x C/6,2x C/5,2x C/4). Growth curves were constructed from measurements on chicks with known hatching date (maximum wing chord, body mass) till day 11. Eighteen adult males and five adult females were captured in Drenthe in the 2012 breeding season, mostly consisting of solitary males. Measurements (body mass, maximum wing chord, tarsus, tarsus+heel, leg width and head+bill are given separately for each individual. None of the birds captured between 27 May and 15 July were moulting. Feathers (2 rectrices, central tertial) were collected for stable isotope analysis. Captured birds resumed their normal activities (singing, feeding) within minutes of their release.